In the case of a semiconductor diode, a current flows when a forward bias is applied, and a current does not flow when a backward bias is applied, so that the current flows only in one direction.
However, in the case of light, it has reciprocity and it is not easy to limit the flow of light to a specific direction or one direction. The reciprocity of light means a symmetry of a wave transmitted between two points in a space, wherein the same wave passing through the same medium moves in the same way regardless of the direction of movement.
FIG. 4 shows a symmetry-dependent T matrix and its example structures in the conventional meta-materials (see, C. Menzel et al., PRA 82, 053811 (2010)). The structure of the meta-material for symmetric transmission has z-axis (a light propagation direction) symmetry, and the symmetry about the z-axis needs to be broken for the asymmetric transmission.
A photonic diode has characteristics that it has structure for the asymmetric transmission.
In the case of a circular polarization, the photonic diode may be fabricated in a two-dimensional structure. However, in the case in a linear polarization, since the photonic diode requires a three-dimensional structure, the manufacture thereof was not easy.
Further, as researches in the field of a nano-technology have progressed actively at the inside and outside of the country, various applications for commercialization of the nano-technology in various fields have been made in various fields such as semiconductors, displays, optical devices and functional devices. A nano-patterning technology for manufacturing nano-level materials is a pre-requisite for meeting the practical requirement of commercialization of the nano-technology, and an electron beam lithography is actively studied as a main means of nano-patterning. Using the electron beam lithography technology, the three-dimensional electron beam patterning is possible.